1. Field of the Invention
The present invention relates to a device for controlling a brake of a vehicle such as an automobile, and more specifically, to such a device that increases braking forces applied to wheels of a vehicle for avoiding a collision with an obstacle in front of the vehicle.
2. Description of Prior Art
For safety in driving a vehicle, a device for detecting a risk (possibility) of a collision with an obstacle has been equipped on an individual vehicle. Such a device monitors a distance and a velocity of an obstacle (e.g. another vehicle) in the vehicle's driving course relative to the vehicle with e.g. a radar sensor system provided near the front or head lights of the vehicle. When the distance (relative distance) between the vehicle and obstacle getting close to each other at a relative velocity becomes too short, namely when a risk of a collision is detected, the device will warn the driver of the collision, prompting him to slow down his vehicle by a braking operation and/or to change the vehicle's driving course by a steering operation. Some of recent devices make a vehicle slow down in response to a detection of a possibility of a collision by applying a braking force to the vehicle automatically (irrespective of the driver's braking operation) or by boosting up the factor of a braking pressure to a brake pedal depression. These braking controls are often referred to as “Precrash Safety Control”. Examples of such devices are seen in Japanese Laid-Open Patent Publications (JP) Nos. 60-91500, 6-298022, 10-297452 11-203598, and 2002-59820.
Braking operation in the precrash safety control should be started when a relative distance is long enough for slowing down, and preferably, stopping a vehicle before colliding against an obstacle. As readily understood, the longer relative distance is required at the higher relative velocity for avoiding a collision with an obstacle. Further, JP 6-2988022, filed by the same assignee as the present application, has revealed that a distance required for avoiding a collision with an obstacle also depends upon ways of avoiding a collision. In this publication, using a simplified model, calculated as a function of a relative velocity were the shortest relative distances required for avoiding the collision only by braking operation (collision Avoidable Limit by Braking: BAL) and only by steering operation (collision Avoidable Limit by Steering: SAL), respectively. Then, it has been found that, qualitatively, BAL is shorter than SAL in the range of low relative velocity and vice versa in the range of high relative velocity, showing that, at a high relative velocity, a collision can be avoided by appropriate steering operation even when the relative distance falls below BAL.
It should be also considered in determining the timing of starting the precrash safety control that the braking operation may reduce a lateral or cornering force available on a wheel, so that premature and excessive braking operation would deteriorate the steering controllability or the course-tracking ability of a vehicle, leading to restriction of the driver's steering action for avoiding an obstacle. Thus, for fully ensuring the steering controllability, in the device shown in JP6-298022, automatic braking operation is executed only when a relative distance is shorter than both BAL and SAL, and thereby the driver can steer his vehicle as long as any effective way of avoiding a collision is available. JP 11-203598 modified the device in the aforementioned publication such that, in the range of low relative velocity, automatic braking operation is started at a certain braking level when a relative distance falls below SAL.
In the devices as described above, a collision would be effectively avoided as long as an appropriate steering operation is executed or executable by the driver. However, in the range of high relative velocity, the automatic braking operation, executed after the relative distance falls below SAL, would be too late to stop a vehicle before colliding against an obstacle because SAL is often far lower than BAL. In the range of low relative velocity, the stronger braking force or the rapider braking operation would be required for the higher relative velocity at which the relative distance falls below BAL. Strong or rapid braking operation often causes a rear-end collision with a following vehicle.
Further, in the aforementioned publication, no braking operation is executed unless the steering operation is ineffective for avoiding a collision because braking operation would reduce the steering controllability. However, the braking operation is not always harmful in steering operation for avoiding a collision. For instance, when a relative distance falls in the range below BAL but above SAL at a high relative velocity, the precrash safety braking operation at a certain level would reduce the relative velocity, resulting in the increase of the time for a vehicle to reach to an obstacle, so that the driver may steer the vehicle more easily for avoiding the obstacle. At a low relative velocity, theoretically, the lowering of the relative velocity by the precrash safety braking could recover the ability of steering operation for avoiding a collision.
It should be also noted that the steering controllability may be ensured unless excessive braking forces are applied on wheels. A lateral force generated on a wheel varies within the maximum available frictional force on the wheel, depending on a steering angle controlled by the driver's operation. Thus, a braking force may be increased until the resultant force of the lateral and braking forces reaches to the maximum frictional force, while ensuring a cornering force requested by the driver through his steering handle, i.e. the steering controllability.
Accordingly, the precrash safety control may be improved to function more effectively for avoiding a collision with an obstacle and/or reducing damage upon collision, considering the relation between the steering controllability and effects of the braking operation.